Method of making halogen doped optical element

ABSTRACT

A method of forming an optical element is provided. The method includes producing silica-based soot particles using chemical vapor deposition, the silica-based soot particles having an average particle size of between about 0.05 μm and about 0.25 μm. The method also includes forming a soot compact from the silica-based soot particles and doping the soot compact with a halogen in a closed system by contacting the silica-based soot compact with a halogencontaining gas in the closed system at a temperature of less than about 1200° C.

This application claims the benefit of priority to U.S. ProvisionalApplication No. 62/084,846 filed on Nov. 26, 2014 the content of whichis incorporated herein by reference in its entirety.

FIELD

The present disclosure relates generally to optical elements, and inparticular, to methods for forming optical elements from doped silicaglass articles.

BACKGROUND

Optics, particularly reflective optics, are an important part ofelements employed in Extreme Ultra-Violet (EUV) lithography. Theseelements are used with extreme ultraviolet radiation to illuminate,project, and reduce pattern images that are utilized to form integratedcircuit patterns. The use of extreme ultraviolet radiation is beneficialin that smaller integrated circuit features can be achieved; however,the manipulation of the radiation in this wavelength range raiseschallenges.

In these and similar applications, low thermal expansion glass, such assilica-titania glass, is currently being used for making projectionoptics. In contrast to other materials, low thermal expansion glassprovides improved polishability, improved coefficient of thermalexpansion (CTE) control, and improved dimensional stability. However, asthe development of these and similar applications advances, the demandfor improved material characteristics grows.

Ultra-low expansion (ULE) glasses and EUV lithographic elements havetraditionally been made by chemical vapor deposition (CVD) processes. InCVD processes, high purity precursors are injected into flames to formfine particles that are directed toward the surface of growing glass. Inthe process, the glass is formed in layer deposits. One limitation ofULE glass made in accordance with CVD processes is that the resultingglass contains striae. Striae are compositional non-uniformities whichadversely affect optical transmission in elements made from the glass.Striae result from thermal variations in the flame during the formationof the fine particles, and are also a result of thermal variations ofthe growing glass as the fine particles are deposited. Striae result inalternating thin layers of different CTE and therefore alternatingplanes of compression and tension between the layers. Striae in ULEglass are evident in the direction parallel with the top and bottom ofthe glass.

In some cases, striae have been found to impact surface finish at anangstrom root mean square (rms) level in reflective optical elements,which can adversely affect the polishability of the glass. Polishingglass having striae results in unequal material removal and unacceptablesurface roughness which can present problems for stringent applicationslike EUV lithography elements. For example, it may create amid-frequency surface structure that may cause image degradation inmirrors used in the projection systems for EUV lithography. Whileattempts have been made to modify and control aspects of the CVDprocesses to reduce striae, the fact that the method forms ULE glass inlayer deposits at least partially contributes to the formation ofstriae.

Conventional optical fibers typically have a silica-based glass coreregion surrounded by a silica-based glass cladding. Some optical fibersinclude a core region that is doped with a dopant, such as GeO₂,suitable for raising the refractive index of the core region. Otheroptical fibers include a pure silica core region and at least onecladding region that is doped with a dopant, such as fluorine, suitablefor lowering the refractive index of the doped cladding. The indexdifference between the core and the doped cladding is necessary tocreate a light guide wherein propagating light is generally confined tothe core region. However, the optical loss, or attenuation, of theoptical fiber having Ge-doped glass in the core region is higher thanthe attenuation expected in pure silica glass, and doping the claddingregion affects the viscosity of the cladding glass. A viscosity mismatchbetween the core and cladding regions results in the region of theoptical fiber having the higher viscosity bearing more tension duringthe process in which an optical fiber is drawn from an optical fiberpreform. The resulting stress may be retained within the optical fiberas residual stress which may become “frozen” in the fiber upon coolingfrom the draw temperature and may contribute to increased attenuation ofthe resulting optical fiber.

Improved material characteristics and/or performance of these opticalelements may be achieved by forming the optical elements fromsilica-based glass articles that include dopants different from, or inaddition to, conventional dopants. However, limitations on dopingefficiency and doping levels with these different, or additional,dopants make it difficult to achieve such improved materialcharacteristics and/or performance.

SUMMARY

According to an embodiment of the present disclosure, a method offorming an optical element is provided. The method includes producingsilica-based soot particles using chemical vapor deposition, thesilica-based soot particles having an average particle size of betweenabout 0.05 μm and about 0.25 μm. The method also includes forming a sootcompact from the silica-based soot particles and doping the soot compactwith a halogen in a closed system by contacting the silica-based sootcompact with a halogen-containing gas in the closed system at atemperature of less than about 1200° C.

According to another embodiment of the present disclosure, a method offorming an optical element is provided. The method includes producingsilica-based soot particles comprising silica and titania using chemicalvapor deposition, the silica-based soot particles having an averageparticle size of between about 0.05 μm and about 0.25 μm. The methodalso includes forming a soot compact from the silica-based sootparticles and doping the soot compact with a halogen in a closed systemby contacting the silica-based soot compact with a halogen-containinggas in the closed system at a temperature of less than about 1200° C.The method further includes consolidating the soot compact in the closedsystem to form a glass article by simultaneously increasing thetemperature in the closed system and decreasing the concentration of thehalogen-containing gas in the closed system.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments as described herein, including the detailed descriptionwhich follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understanding the natureand character of the claims. The accompanying drawings are included toprovide a further understanding, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiment(s), and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be understood more clearly from the followingdescription and from the accompanying figures, given purely by way ofnon-limiting example, in which:

FIG. 1 is a top view illustrating a mold assembly in accordance withembodiments of the present disclosure;

FIG. 2 is a schematic view of a closed system in accordance withembodiments of the present disclosure;

FIG. 3 is a graph depicting dopant concentration vs. distance from theouter surface of a glass article in accordance with embodiments of thepresent disclosure;

FIG. 4 is a graph depicting dopant concentration vs. distance from theouter surface of a glass article in accordance with embodiments of thepresent disclosure;

FIG. 5 is a graph depicting dopant concentration vs. distance from theouter surface of a glass article in accordance with embodiments of thepresent disclosure; and

FIG. 6 is a schematic depiction of soot preform deposition via an OVDprocess.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiment(s), anexample(s) of which is/are illustrated in the accompanying drawings.Whenever possible, the same reference numerals will be used throughoutthe drawings to refer to the same or like parts.

The singular forms “a,” “an” and “the” include plural referents unlessthe context clearly dictates otherwise. The endpoints of all rangesreciting the same characteristic are independently combinable andinclusive of the recited endpoint. All references are incorporatedherein by reference.

The present disclosure is described below, at first generally, then indetail on the basis of several exemplary embodiments. The features shownin combination with one another in the individual exemplary embodimentsdo not all have to be realized. In particular, individual features mayalso be omitted or combined in some other way with other features shownof the same exemplary embodiment or else of other exemplary embodiments.

Optical elements and methods of forming such optical elements areprovided herein. As used herein, the terms “optic” and “optical element”denote a transparent glass article that can be formed into a reflectiveor transmissive element that is intended to be used to reflect, transmitor guide light. The methods described herein facilitate forming largeand uniform glass optical elements. The methods also provide forefficient use of dopant material during the formation of the opticalelements.

The methods facilitate forming optical elements capable of use inphotolithography that are large, uniform and highly polishable. Suchelements may have a near-zero thermal expansion over a wide operationaltemperature range, such as between about 20° C. and about 30° C., andmay have a slope of CTE versus temperature at 20° C. of less than about1.0 ppb/K². A material that has near-zero thermal expansion is one thatundergoes little or no dimensional change in response to changingtemperature. As compared to optical elements capable of use inphotolithography formed using a CVD process, elements made in accordancewith the methods disclosed herein include lower variations in titaniaand halogen composition compared to the local average titania andhalogen levels.

The methods described herein also facilitate forming optical elementscapable of use as optical fibers. Such elements include a silica-basedglass core region which may have a chlorine content of greater thanabout 1.5 wt. %. The core region is surrounded by at least onesilica-based glass cladding region which may include pure silica orwhich may include silica doped with a dopant, such as fluorine, suitablefor lowering the refractive index of the cladding. Where the opticalfiber includes more than one cladding region, at least one of thecladding regions may include silica doped with a dopant such asfluorine. As compared to optical fibers which include a core region thatis doped with GeO₂, the optical fibers described herein have a lowerattenuation.

The method may include producing silica-based soot particles.Silica-based soot particles as described herein may be a by-product ofhigh purity fused silica glass making processes which may include, butare not limited to, conventional CVD processes for making optical fiberpreforms, such as outside vapor deposition (OVD) and vapor axialdeposition (VAD) processes. Silica-based soot particles may also becollected from silica soot generation systems in which the silica-basedsoot particles are collected in a loose state. By “loose state” it ismeant that the particles are not contacted with a collecting surfaceprior to being cooled and are not contacted with a collecting surfaceprior to substantially all of the precursor material being consumed bythe flame. The silica-based soot particles may have an average particlesize of between about 0.05 μm and about 0.25 μm. The particulate surfacearea of the silica-based soot particles may be greater than about 10m²/g, or greater than about 15 m²/g, or greater than about 20 m²/g, orgreater than about 50 m²/g, or even greater than about 100 m²/g.

In addition to silica, the silica-based soot particles may includebetween about 1.0 wt. % and about 14 wt. % titania, or between about 5.0wt. % and about 10 wt. %. titania. The silica-based soot particles mayinclude about 8.0 wt. % titania. The silica-based soot particles mayalso include between about 1.0 wt. % and about 10 wt. % of one or moreadditives, or between about 1.0 wt. % and about 6.0 wt. % of one or moreadditives. The one or more additives may include, but are not limitedto, boron containing compounds, fluorine containing compounds, chlorinecontaining compounds, phosphorous containing compounds and mixturesthereof. Where titania is included, the silica-based soot particles mayhave a binary composition of silica and titania or may have a ternarycomposition of silica, titania and an additive. The silica-based sootparticles may also have a composition of silica, titania and a pluralityof additives.

The method may also include forming a soot compact by pressingsilica-based soot particles to form the soot compact. As is shown inFIG. 1, a mold assembly 10 having a mold cavity 12 may be filled withthe silica-based soot particles. The mold cavity 12 may be of any shape,such as, but not limited to, round or elliptical, and may be selectedbased on a predetermined shape of the soot compact. The mold assembly 10may have the same shape as the mold cavity 12. Alternatively, the moldassembly 10 may have a different shape than the mold cavity 12. Forexample, if the mold cavity 12 has an elliptical shape, the moldassembly 10 may also have an elliptical shape. Or, as shown in FIG. 1,the mold assembly 10 may instead be rectangular. The mold cavity 12 maybe filled with the silica-based soot particles. The design of the moldassembly 10 should be suitable to resist deformation in response topressure exerted by the silica-based soot particles as the particles arepressed to form a soot compact. Embodiments of the present disclosurefacilitate formation of large soot compacts, from which large opticalelements, such as those used in EUV lithography, may be formed or fromwhich large optical fiber preforms may be formed.

A pressing mechanism (not shown), such as a hydraulic press, may have apressing plate that may be brought into contact with the silica-basedsoot particles in the mold cavity 12. The pressing plate may be shapedto enter the mold cavity 12 to press the silica-based soot particleswithout contacting the walls of the mold cavity 12. To prevent contactwith the walls of the mold cavity 12, the pressing plate may be sizedsuch that a gap exists between the outside edge of the pressing plateand the walls of the mold cavity 12. For example, where both the moldcavity 12 and the pressing plate are elliptical in shape, a gap betweenthe walls of the mold cavity 12 and the pressing plate may exist aroundthe entire circumference of the pressing plate. The gap may beapproximately equal at all points around the outside edge of thepressing plate, and may be less than about 0.10 inches, or between about0.005 inches and about 0.10 inches, or even between about 0.005 inchesand about 0.06 inches. In addition to preventing contact of the pressingplate with the walls of the mold cavity 12, the gap may also serve as apassage for the escape of gas while pressing the silica-based sootparticles to form the soot compact.

Once in contact with the silica-based soot particles, the pressing platemay be moved at a rate of less than about 10 mm per second. For example,the pressing plate may be moved at a rate of between about 1.0 mm persecond and about 10 mm per second, or between about 1.0 mm per secondand about 5.0 mm per second, or at a rate of about 3.0 mm per second.The pressing mechanism may assert a pressure on the silica-based sootparticles of less than about 1000 psi, or less that about 250 psi, oreven less than about 200 psi. For example, the pressing mechanism mayassert a pressure on the silica-based soot particles of between about100 psi and about 1000 psi, or even between about 100 psi and about 500psi. Pressure may be applied in an axial direction to form a disc shapedsoot compact.

The mold cavity 12 may be of any volume, and may be large enough to forma soot compact having a mass of greater than about 20 kg, greater thanabout 30 kg, or even greater than about 120 kg or more. The volume ofthe mold cavity 12 may be determined based on the size of the sootcompact and the size of the intended optical element. Depending on thepredetermined size of the soot compact, the mold cavity 12 should belarge enough to accommodate an unpressed volume of loose silica-basedsoot particles. For example, in some cases, the volume of the moldcavity 12 may be about three times to about six times the volume of thesoot compact.

Alternatively, the mold assembly 10 may further include a subassemblydetachably connected to the mold assembly 10 to provide additionalvolume to accommodate loose silica-based soot particles prior topressing the silica-based soot particles to form the soot compact. Thesubassembly may include a cavity having a top opening and a bottomopening, the top and bottom openings having the same shape as the moldcavity 12. When attached to the mold assembly 10, the subassembly cavitymay be aligned with the mold cavity 12 to extend the volume of the moldcavity 12. In such a design, the mold cavity 12 may have a volumeapproximately equal to the predetermined size of the soot compact, andthe combination of the volume of the mold cavity 12 and the volume ofthe subassembly cavity may be about three times to about six times thevolume of the soot compact.

The pressing mechanism may include an ultrasonic gauge used to determinethe density of the soot compact. Once the soot compact is pressed to apredetermined density, as measured by the ultrasonic gauge, movement ofthe pressing plate may be stopped. The soot compact may be pressed to adensity of between about 0.50 g/cc and about 1.20 g/cc, or between about0.70 g/cc and about 1.10 g/cc, or even between about 0.80 g/cc and about1.00 g/cc. Once the soot compact is pressed to a predetermined density,the pressing plate may then be released by moving the pressing plate outof contact with, and away from, the soot compact. Alternatively, thepressing plate may not be released and the soot compact may bemaintained under pressure for a period long enough to allow the sootcompact to relax into a compressed state. The period to allow the sootcompact to relax into a compressed state may be, for example, greaterthan about 10 minutes, or between about 10 minutes and about 48 hours.The period to allow the soot compact to relax into a compressed statemay be, for example, about 5.0 hours.

Pressing the silica-based soot particles to form the soot compact maynot require the addition of heat and may be performed at roomtemperature. Also, pressing the silica-based soot particles to form thesoot compact may not include intentionally adding a binder or liquid,such as water, to the silica soot particles.

Once the soot compact is formed, the soot compact may be removed fromthe mold cavity 12. The soot compact may be heated to a temperatureabove about 700° C. to induce a small amount of shrinkage of the sootcompact, which may permit movement of the soot compact out of the moldcavity 12. Alternatively, the mold may be a segmented assembly, andremoval of the soot compact may include disassembling the mold.

As an alternative to pressing silica-based soot particles to form a sootcompact, the method may include forming a soot compact using any offlame combustion methods, flame oxidation methods, flame hydrolysismethods, OVD, IVD (inside vapor deposition), VAD, double cruciblemethod, rod-in-tube procedures, cane-in-soot method, and doped depositedsilica processes. In such methods, silica-based soot particles areproduced by combusting a silica precursor in a flame and depositing thesilica-based soot particles on a rotating bait rod. The silicaprecursors may be, for example, OMCTS (octamethylcyclotetrasiloxane) orSiCl₄.

By way of example and not intended to be limiting, formation of a sootpreform according to an OVD method is illustrated in FIG. 6. As shown,soot preform 20 is formed by depositing silica-based soot particles 22onto the outer surface of a rotating and translating bait rod 24. Baitrod 24 may be tapered. The silica-based soot particles 22 are formed byproviding a glass/soot precursor 28 in gaseous form to a flame 30 of aburner 26 to oxidize the precursor 28. Fuel 32, such as methane (CH₄),and combustion supporting gas 34, such as oxygen, are provided to theburner 26 and ignited to form the flame 30. Mass flow controllers,labeled V, meter the appropriate amounts of glass/soot precursor 28,fuel 32 and combustion supporting gas 34, all preferably in gaseousform, to the burner 26. The glass/soot precursor 28 is a glass formercompound and is oxidized in the flame 30 to form the generallycylindrically-shaped soot region 23, which may correspond to the core ofan optical fiber preform.

The method may also include doping the soot compact which may includecontacting the soot compact with a dopant containing gas. The dopantcontaining gas may include, but is not limited to, a halogen-containinggas such as a fluorine-containing gas, a chlorine-containing gas or abromine-containing gas. The fluorine-containing gas may be, but is notlimited to, F₂, C₂F₆, CF₄, SF₆ and SiF₄, and combinations thereof. Thechlorine-containing gas may be, but is not limited to, SiCl₄, Cl₂ andPOCl₃. The bromine-containing gas may be, but is not limited to, SiBr₄.

As shown in FIG. 2, the soot compact may be doped in a closed system100. As used herein, the term “closed system” denotes a system which canenclose the entire soot compact and which limits gases within the closedsystem from flowing out of the system, and which limits ambient air fromflowing into the system. The closed system 100 as shown in FIG. 2 is asealed reaction chamber 112 having an interior 114 of the sealedreaction chamber 112. The sealed reaction chamber 112 may be, forexample, a furnace in which later heat treatment of the soot compact maybe performed. The temperature in the reaction chamber 112 during dopingmay be less than about 1200° C. For example, the temperature in thereaction chamber 112 during doping may be between about 300° C. andabout 1200° C., or between about 850° C. and about 1100° C. Doping at atemperature of less than about 1200° C. is believed to promote uniformhalogen doping of the soot compact, particularly in large soot compactssuch as those used to form optical elements for EUV lithography andoptical fiber preforms. A heating device 116 partially or fullysurrounds a portion of the sealed reaction chamber 112. The heatingdevice 116 may be, for example, an electrical coil that, in combinationwith a susceptor, forms an inductive heater. Alternatively, the heatingdevice 116 may be an electrical resistance heater or any other suitableheating device that can provide sufficient heat to perform the method asdescribed herein.

The closed system 100 may include a first gas source 128 in fluidconnection with the reaction chamber 112 through a first inlet 120 fromwhich a dopant containing gas may be introduced into the reactionchamber 112. The closed system may also include an outlet 122. A fluidcontrol system may also be associated with the closed system 100 and maycontrol the flow of gas in the closed system 100. The fluid controlsystem may monitor the gas composition in the closed system 100 andmaintain a predetermined dopant gas composition in the closed system100. The predetermined dopant gas composition in the closed system 100may be greater than about 90%, or greater than about 95%, or evengreater than about 98% of the total gas composition of the closed system100. Additionally, or in the alternative, the fluid control system maymonitor the dopant gas partial pressure in the closed system 100, andmaintain a predetermined dopant gas partial pressure in the closedsystem 100. The predetermined dopant gas partial pressure in the closedsystem 100 may be less than about 0.90 atm, or less than about 0.50 atm.The predetermined dopant gas partial pressure in the closed system 100may be between about 0.10 atm and about 0.50 atm.

The closed system 100 may include a second gas source 130 in fluidconnection with the reaction chamber 112 through a second inlet 124.Between the second gas source 130 and the second inlet 124, a one-wayvalve 135 permits gas flow from the second gas source 130 into thereaction chamber 112. Maintaining a predetermined dopant gascomposition, and/or maintaining a predetermined dopant gas partialpressure, may include bleeding dopant containing gas from the second gassource 130 into the reaction chamber 112. Without being limited by anyparticular theory, at least some amount of the dopant containing gaswill be consumed in the closed system 100 as the dopant is absorbed bythe soot compact 20. The fluid control system may measure theconsumption of the dopant containing gas as a decrease in gascomposition and/or as a decrease in partial pressure in the reactionchamber 112. In order to maintain the predetermined dopant gascomposition, and/or the predetermined dopant gas partial pressure, thefluid control system may open the one-way valve 135 to permit dopantcontaining gas flow into the reaction chamber 112 until thepredetermined dopant gas composition, and/or the predetermined dopantgas partial pressure is restored. Once the predetermined dopant gascomposition, and/or the predetermined dopant gas partial pressure isrestored, the fluid control system may close the one-way valve 135.

Doping the soot compact may also include pulling a vacuum on the closedsystem 100 prior to doping the soot compact and maintaining vacuumconditions while doping the soot compact. When under vacuum, totalpressure in the reaction chamber 112 may be greater than about 0.50 atm.For example, the total pressure in the reaction chamber 112 may bebetween greater than about 0.25 atm, such as between about 0.25 atm andabout 2.0 atm, or between about 0.5 atm and about 5.0 atm, or betweenabout 0.90 atm and 2.0 atm. According to embodiments of the presentdisclosure, total pressure in the reaction chamber 112 may be greaterthan 2.0 atm, such as between about 2.0 atm and about 30 atm. Generally,where the intent is to achieve uniform dopant content in the sootcompact, doping the soot compact may be performed at lower pressuressuch as between about 0.25 atm and about 1.0 atm. However, where theintent is to achieve high dopant content or both high dopant content anduniform dopant content in the soot compact, doping the soot compact maybe performed at higher pressures such as greater than about 0.90 atm.

Prior to doping the soot compact, the method may also include drying thesoot compact. Drying the soot compact may include a plurality of dryingcycles which include filling the reaction chamber 112 with a gascomposition having less than about 50% of a drying gas, and evacuatingthe gas composition after a predetermined drying period. Alternatively,the gas composition may have less than about 10% of a drying gas, oreven less than about 5.0% of a drying gas. The remainder of the gascomposition may be helium. The predetermined drying period may begreater than about 10 minutes and may be, for example, between about 10minutes and about 1.0 hour. The drying gas may be, but is not limitedto, chlorine or chlorine containing gases and carbon monoxide. Dryingthe soot compact may prepare the soot compact for doping by removingmoisture and hydroxyl groups from the soot compact, and also by removingtransition metals or alkali metal components from the soot compact. Whenthe drying cycles are complete, the reaction chamber 112 may beevacuated prior to introduction of a dopant containing gas into thereaction chamber 112.

The method may also include consolidating the doped soot compact to forma glass article. The doped soot compact may be heated to a sinteringtemperature between about 1200° C. and about 1650° C. and maintained atthe sintering temperature until the soot compact is consolidated into aglass article. As explained above, the closed system 100 may be afurnace in which the doped soot compact may be consolidated, or thedoped soot compact may be moved from the closed system 100 describedabove to a furnace where the doped soot compact may be consolidated.

Consolidating the doped soot compact to form a glass article may furtherinclude simultaneously increasing the temperature of the closed system100 and decreasing the concentration of the dopant containing gas in theclosed system 100. The concentration of dopant containing gas may bedecreased with the increase of furnace temperature in accordance withthe following relation:

$\begin{matrix}{y_{II} = {y_{I,{dop}}{{Exp}\left\lbrack {{- 21741}\left( {\left( \frac{1}{T_{I,{dop}}} \right) - \left( \frac{1}{T_{II}} \right)} \right)} \right\rbrack}}} & (1)\end{matrix}$

wherein: T_(I,dop) is the temperature in the closed system during dopingof the soot compact with a halogen;

T_(II) is the temperature in the closed system during consolidation ofthe soot compact;

y_(I,dop) is the mole fraction of the halogen-containing gas in theclosed system during doping of the soot compact with halogen; and

y_(II) is the maximum halogen-containing gas mole fraction in the closedsystem during consolidation of the soot compact at T_(II).

The method may also include oxygenating the soot compact by contactingthe soot compact with an oxygen-containing gas. The soot compact may beoxygenated at any time, including prior to or during drying of the sootcompact, prior to or during doping of the soot compact, and prior to orduring consolidation of the soot compact. Oxygenating the soot compactmay be performed at a temperature of between about 1000° C. and about1300° C. The soot compact may be oxygenated in an oxygen containingatmosphere with oxygen diffusing into the soot compact and reacting withtrivalent titanium (Ti³⁺) to lower oxidation states of titanium and toconvert such titanium to tetravalent titanium (Ti⁴⁺). Such oxygenatingmakes the consolidated glass article colorless and is believed toprevent the occurrence of a bluish-black discoloration of theconsolidated glass article.

The method may also include annealing the glass article. Followingconsolidation, the reaction chamber 112 may be cooled to a holdingtemperature of between about 900° C. and about 1100° C. for a holdingperiod of at least about 30 minutes, for example, between about 30minutes and about 10 hours. After completion of the holding period, thereaction chamber 112 temperature may be decreased to a predeterminedtemperature, between about 700° C. and about 850° C., at a rate of lessthan about 10° C. per hour. For example, the rate may be between about0.10° C. per hour and about 10° C. per hour, or between about 0.10° C.per hour and about 5.0° C. per hour, or even between about 0.10° C. perhour and about 1.0° C. per hour. Once the predetermined temperature isreached, heat from the heat source may be removed, and the reactionchamber 112 may be allowed to cool to ambient temperature. Afterannealing, the fictive temperature of the glass article may be less thanabout 1100° C., or less than about 1000° C., or less than about 900° C.,or even less than about 800° C.

The glass article may be annealed in the closed system 100, as describedabove, or the glass article may be moved from the closed system 100 andannealed in a separate vessel, such as a furnace. Similarly, where thedoped soot compact is removed from the closed system 100 andconsolidated in a separate furnace, the glass article may be annealed inthe consolidation furnace, or may be moved from the consolidationfurnace and annealed in a separate vessel, such as a separate furnace.

The method may also include processing the glass article to form opticalelements. Once the glass article has been cooled to ambient temperature,the glass article may be cut, cored, reflowed into a target shape, orotherwise processed into shapes that are suitable for making opticalelements. Such processing, in addition to cutting or coring, may includeetching, additional thermal treatments, grinding, polishing, applyingselected metals to form a mirror, reflowing, and such additionalprocessing as may be necessary to form the desired optical element.According to embodiments of the present disclosure, additionalsilica-based soot may be deposited onto the glass article to form atleast one optical fiber cladding region using the same method asexplained above with respect to the core of an optical fiber preform.The at least one optical fiber cladding region may optionally be dopedwith a halogen using a halogen-containing dopant gas as described hereinusing the same doping steps as described herein, or using doping stepsknown in the art.

The optical elements capable of use in photolithography disclosed hereinmay be formed from a fluorine-doped silica-titania glass article. Thedoped glass article may include between about 0.50 wt. % and about 2.0wt. % fluorine and the variation of fluorine concentration through thethickness of the doped glass article may be less than about 0.20 wt. %.The doped glass article may also include between about 1.0 wt. % andabout 12 wt. % titania, or between about 5.0 wt. % and about 10 wt. %titania. The variation of titania concentration through the thickness ofthe doped glass article may be less than about 0.10 wt. %, and the dopedglass article may be uniform and substantially free of striae. Suchuniformity renders the doped glass article polishable, which in turnfacilitates processing of the doped glass article to form the opticalelements disclosed herein. The optical elements may have a near-zerothermal expansion over a wide operational temperature range, such asbetween about 20° C. and about 30° C., and may also have a slope of CTEversus temperature at 20° C. of less than about 1.0 ppb/K². The slope ofCTE versus temperature at 20° C. may be less than about 0.80 ppb/K², oreven less than about 0.60 ppb/K².

The optical elements disclosed herein may be photomask blanks orprojection optic mirror substrates employed in EUV lithography. Thedoped glass article disclosed herein may also be used to form thecritical zone of large mirrors used in EUV lithography. The doped glassarticle may be shaped and fusion bonded into a cavity formed in thecritical zone in a larger undoped glass article.

Alternatively, the doped glass article disclosed herein may be a sootblank which may be used to form the core of an optical fiber preform, ormay be an optical fiber preform that may be drawn into an optical fiber.The optical fiber may be formed from a chlorine-doped silica glassarticle as disclosed herein. Such optical fiber may have a core regionhaving a chlorine content of greater than about 1.5 wt. %. For example,the core region of the optical fiber may have a chlorine content ofbetween about 1.5 wt. % and about 4.75 wt. %, or between about 1.5 wt. %and about 4.5 wt. %, or between about 1.5 wt. % and about 4.0 wt. %, orbetween about 1.5 wt. % and about 3.0 wt. %. The core region of theoptical fiber may have a chlorine content of between about 1.75 wt. %and about 4.5 wt. %, or between about 1.75 wt. % and about 4.0 wt. %, orgreater than about 1.5 wt. %, or greater than about 1.75 wt. %, orgreater than about 2.0 wt. %, or greater than about 2.25 wt. %.

Additionally, the optical fiber may be formed from a silica glassarticle doped with both chlorine and fluorine. In addition to a coreregion having a chlorine content such as described above, such opticalfiber may have at least one cladding region having a fluorine content ofbetween about 0.10 wt. % and about 0.50 wt. %. For example, the at leastone cladding region may have a fluorine content of between about 0.15wt. % and about 0.45 wt. %, or between about 0.20 wt. % and about 0.40wt. %.

The optical fiber described herein may also be described using therelative refractive index of various regions of the optical fiber. Asused herein the term “relative refractive index” or “relative refractiveindex percent” is defined as:

Δ%=100×(n _(i) ² −n _(c) ²)/2n _(i) ²

where n_(c) is the refractive index of undoped silica and n_(i) is theaverage refractive index at point i in the particular region of theoptical fiber.

As further used herein, the relative refractive index is represented byΔ and its values are given in units of “%”, unless otherwise specified.The terms Δ, %Δ, Δ%, delta index, percent index, percent delta index and% can be used interchangeably herein. In cases where the refractiveindex of a region is less than the refractive index of undoped silica,the relative index percent is negative and is referred to as having adepressed region or depressed index.

The optical fiber formed from the chlorine-doped silica glass article asdisclosed herein may have a core region having a relative refractiveindex between about 0.08% and about 0.30%. For example, the core regionmay have a relative refractive index of between about 0.10% and about0.25%, or between about 0.12% and about 0.20%, or even between about0.14% and about 0.18%.

Additionally, the optical fiber formed from a silica glass article dopedwith both chlorine and fluorine may have at least one cladding regionhaving a relative refractive index between about 0% and about 0.25%, orbetween about −0.05% and about −0.20%, or between about −0.10% and about−0.20%.

Methods described herein enable more efficient use of dopant containinggases, which in turn thus reduce the overall costs associated withforming doped glass articles as described herein. Conventional dopingprocesses flow dopant containing gases over a soot blank in an opensystem. The utilization efficiency of such processes, defined as theamount (in wt. %) of the dopant provided in the dopant containing gasdivided by the amount (in wt. %) of the dopant in the resulting dopedglass article, is typically utilize only about 10-25%. In suchprocesses, between about 75-90% of the dopant gas is discarded after thedoping process is complete. In contrast, performing a doping process inclosed systems according to embodiments of the present disclosureutilizes or conserves greater than about 90% of the dopant gas. Theclosed system eliminates the loss of dopant gas experienced during adoping process in an open system and also allows for the recovery ofdopant gas once the doping process is complete.

EXAMPLES

Embodiments of the present disclosure are further described below withrespect to certain exemplary and specific embodiments thereof, which areillustrative only and not intended to be limiting.

Example 1

To produce a baseline dopant profile, a silica soot compact having adiameter of about 14 cm was heat treated in a furnace under a heliumatmosphere. The soot compact was held isothermally at a temperature ofabout 1145° C. for a period of about 4.0 hours. For about 1.0 hour ofthe about 4.0 hour period, a gas flow having about 2.0% chlorine wasflowed through the furnace to dry the soot compact of moisture andhydroxyl groups, and to remove any transition metal contaminants. Whilemaintaining the furnace temperature at about 1145° C., a gas flow havingabout 50% silicon tetrafluoride (SiF₄) was then flowed through thefurnace for a period of about 90 minutes to dope the soot compact withfluorine. Subsequently, the furnace temperature was increased from about1145° C. to about 1345° C. over a period of about 1.0 hour under a gasflow of about 6.0% SiF₄, and held for about 30 minutes. The furnacetemperature was then increased to about 1450° C. and the soot compactwas sintered under a helium atmosphere to form a doped glass article. Asillustrated in FIG. 3, the dopant profile of this example shows amonotonic decrease in fluorine content dropping from about 2.0% at thesurface of the glass article to less than about 1.0% near the center ofthe glass article. The non-uniform dopant profile is believed to be aresult of a progressively diffusion limited reaction. It is believedthat the reaction of fluorine at 1145° C. is sufficient to increase thedensification rate at the outer surface of the glass article, andthereby limit diffusivity of the dopant beyond the outer surface of theglass article.

Example 2

To test this belief, a soot compact having a diameter of about 14 cm washeat treated in a furnace under a helium atmosphere. The soot compactwas held isothermally at a temperature of about 1060° C. for a period ofabout 4.0 hours. For about 1.0 hour of the about 4.0 hour period, a gasflow having about 9.0% chlorine was flowed through the furnace to drythe soot compact of moisture and hydroxyl groups, and to remove anytransition metal contaminants. While maintaining the furnace temperatureat about 1060° C., a gas flow having about 25% silicon tetrafluoride(SiF₄) was then flowed through the furnace for a period of about 2.0hours to dope the soot compact with fluorine. Subsequently, the furnacetemperature was increased from about 1060° C. to about 1300° C. over aperiod of about 30 minutes under a gas flow of about 2.0% SiF₄, and heldfor about 15 minutes. The soot compact was then sintered under a heliumatmosphere at a temperature of about 1190° C. for about 12 hours to forma doped glass article. As illustrated in FIG. 4, the dopant profile ofthis example shows uniform fluorine content of about 0.80% from thecenter of the glass article to within about 7.0 mm from the outersurface of the glass article. At the outer surface of the glass article,the fluorine content rises to about 1.0%. It is believed that the lowertemperatures of this example, as compared to Example 1, allowed forsufficient diffusivity of the dopant to improve the uniformity dopedglass article.

Example 3

Attempts were made to better control the process of Example 2 andimprove the uniformity of a doped glass article. A soot compact having adiameter of about 14 cm was heat treated in a closed system under ahelium atmosphere. In this example, the closed system included a sealedfurnace. The soot compact was held isothermally at a temperature ofabout 1060° C. for a period of about 4.0 hours. The system was filledwith a gas having about 50% chlorine to dry the soot compact of moistureand hydroxyl groups, and to remove any transition metal contaminants.After about 20 minutes, the gas was evacuated from the system. Threecycles of filling for about 1.0 minute to about 2.0 minutes, holding forabout 20 minutes, and evacuating for about 10 minutes were performed.Once the final cycle was completed, the system was evacuated for about10 minutes. While maintaining the system temperature at about 1060° C.,the system was filled with a gas having about 100% silicon tetrafluoride(SiF₄) and the system remained closed for about 2.0 hours to dope thesoot compact. Subsequently, a vacuum was pulled on the closed system andthe system temperature was increased from about 1060° C. to about 1350°C. over a period of about 30 minutes and the soot compact was held atabout 1350° C. for about 60 minutes to sinter the soot compact and toform a doped glass article. As illustrated in FIG. 5, the dopant profileof this example shows uniform fluorine content of about 1.90% from thecenter of the glass article to within about 7.0 mm. At the outer surfaceof the glass article, the fluorine content decreases to about 1.10%. Itis believed that the lower temperatures used in Example 2, coupled withthe closed system of this example, allowed for sufficient diffusivity ofthe dopant to improve the uniformity doped glass article.

Example 4

A soot compact was doped with fluorine to show the relationship ofdoping pressure and doping temperature to the dopant content of thedoped glass article. In the present example, a doping temperature ofabout 1100° C. and a doping pressure of about 25 kPa were selected toform a doped glass article having a fluorine content of about 1.5 wt. %.

A 140 gram soot compact was positioned in the center of a hot zone of aclosed system. The closed system was a tube furnace formed form a 48inch long and 3.5 inch diameter alumina tube that was fitted with sealedend caps and a valve on each end of the tube and which had a 24 inch hotzone. A vacuum pump was coupled to the valve on one end of the tubefurnace and a source of SiF₄ was coupled to the valve on the other endof the tube furnace. The tube furnace was also equipped with a vacuumgauge (a Baratron® Direct Pressure/Vacuum Capacitance Manometercommercially available from MKS Instruments, Inc. of Andover, Mass.) andthe pressure in the system was monitored to confirm there was no leakagewhen the valves were closed. With the valves closed, pressure in thetube furnace was reduced to 2.0 kPa and maintained for a period of about6.0 hours.

While under vacuum of less than 2.0 kPa, the temperature of the tubefurnace was raised to a drying temperature of about 1060° C. A sootcompact positioned in the tube furnace was dried by flowing into thefurnace a volume of SiF₄ sufficient to raise the pressure in the tubefurnace to a predetermined target doping pressure, closing the valvesand maintaining the closed system condition for a period of betweenabout 15 minutes and about 30 minutes. The volume of SiF₄ at standardtemperature and pressure (V_(SiF4 STP)) was determined usingV_(SiF4 STP)=(P_(dope)*V_(tube))*(273/T_(dry)) where P_(dope) is thepredetermined target doping pressure, V_(tube) is the volume of the tubefurnace, and T_(dry) is the drying temperature. The SiF₄ was absorbed onthe surface of the soot compact and reacted with SiOH on the surface ofthe soot compact to form HF as a by-product in the closed system.Generally, the pressure in the tube furnace dropped as the absorption ofSiF₄ was greater than the formation of HF. Once the pressure stabilized,the tube furnace was evacuated and then the valves were closed. Whilemaintaining the vacuum of less than 2.0 kPa, the temperature in the hotzone of the tube furnace was raised to a temperature of about 1100° C. Adoping process was initiated by flowing SiF₄ into the tube furnace untila doping pressure of 0.25 kPa was reached. As expected, the pressure inthe tube furnace dropped as SiF₄ was absorbed by the soot compact, andthe target doping pressure was maintained by opening the valve coupledto the source of SiF₄ and flowing more SiF₄ into the tube furnace. Thepressure in the tube furnace was maintained at between about 24 kPa andabout 25 kPa throughout the doping process. The doping process continueduntil the pressure in the tube furnace stabilized. Once the pressure inthe tube furnace stabilized, the temperature of the tube furnace wasraised to a sintering temperature of about 1300° C. to complete theformation of a dense glass article. The glass article was determined tohave a fluorine content of about 1.5 wt. %.

Example 5

A similar process as described in Example 4 was utilized to illustratethat a soot compact can be doped with high levels of chlorine with highdoping efficiency. In this example, a doping temperature of about 1060°C. and a doping pressure of about 101 kPa were selected to form a dopedglass article having a chlorine content of greater than about 2.0 wt. %.The closed system was the same as the closed system of Example 4, exceptthat the closed system includes a 3.5 inch diameter silica tube. Theclosed system was placed in fluid communication with a vaporizerincluding a stainless steel vessel containing SiCl₄. The stainless steelvessel was immersed in an oil bath set at greater than about 57° C. inorder to supply pure SiCl₄ gas at a pressure of about 101 kPa. A supplyof either pure nitrogen or a combination of 2.0 wt. % Cl₂ with a balanceof nitrogen gas was fixtured outside the closed system.

A 140 gram silica soot compact was positioned in the center of a hotzone of the closed system. The pressure in the tube furnace was reducedto 2.0 kPa and, while under vacuum, the temperature of the furnace wasraised to a drying temperature of about 1060° C. The soot compact wasdried by flowing about 1.0 slpm of 2.0 wt. % Cl₂ in nitrogen for about30 minutes through the tube furnace, with both of the valves opened.After drying, the tube furnace was evacuated and then both the valveswere closed. While maintaining the vacuum of less than 2.0 kPa, thetemperature in the hot zone of the tube furnace was raised to atemperature of about 1060° C. The doping process was initiated byreleasing the SiCl₄ at about 57° C. to equilibrate into the systemvolume. As the closed system includes a gas pressure of SiCl₄ that is inequilibrium with the liquid source at the normal boiling point of SiCl₄,a pressure of 101 kPa was maintained during the doping process which wasperformed for about 30 minutes. After the doping process was completedthe temperature of the furnace was raised to a sintering temperature ofabout 1400° C. to complete the formation of a dense glass article. UsingX-ray fluorescence, the glass article was determined to have a chlorinecontent of be about 2.2 wt. %.

Optionally, to increase the material use efficiency of the process, theSiCl₄ remaining in the closed system may be condensed back into thestainless steel vessel for re-use in subsequent processes. Afterformation of the dense glass article, the furnace was allowed to coolwhile the stainless steel vessel was simultaneously cooled to 0° C. inan ice bath. During this cooling period, SiCl₄ was transported back intothe stainless steel vessel. The furnace was then purged of any residuallow level gas using a scrubber. Performing a doping process in closedsystems such as described herein was observed to utilize or conservegreater than about 90% of the dopant gas. While the doping efficiencydescribed in this example relates to a doping process using chlorine, itshould be understood that the increased doping efficiencies describedherein also relate to doping processes using other dopant containinggases.

Example 6

A similar process as described in Example 5 was utilized to illustratedoping a soot compact with even high levels of chlorine whilemaintaining high doping efficiency. In this example, a dopingtemperature of about 1060° C. and a doping pressure of about 101 kPawere selected to form a doped glass article having a chlorine content ofgreater than about 2.0 wt. %. The closed system was the same as theclosed system of Example 4, except that the closed system includes a 3.5inch diameter silica tube. The closed system was placed in fluidcommunication with a vaporizer including a stainless steel vesselcontaining SiCl₄. The stainless steel vessel was immersed in an oil bathset at greater than about 80° C. in order to supply pure SiCl₄ gas at apressure of about 180 kPa. A supply of either pure nitrogen or acombination of 2.0 wt. % Cl₂ with a balance of nitrogen gas was fixturedoutside the closed system.

A 140 gram silica soot compact was positioned in the center of a hotzone of the closed system. The pressure in the tube furnace was reducedto 2.0 kPa and, while under vacuum, the temperature of the furnace wasraised to a drying temperature of about 1060° C. The soot compact wasdried by flowing about 1.0 slpm of 2.0 wt. % Cl₂ in nitrogen for about30 minutes through the tube furnace, with both of the valves opened.After drying, the tube furnace was evacuated and then both the valveswere closed. While maintaining the vacuum of less than 2.0 kPa, thetemperature in the hot zone of the tube furnace was raised to atemperature of about 1060° C. The doping process was initiated byreleasing the SiCl₄ at about 80° C. to equilibrate into the systemvolume. As the closed system includes a gas pressure of SiCl₄ that is inequilibrium with the liquid source at the normal boiling point of SiCl₄,a pressure of 180 kPa was maintained during the doping process which wasperformed for about 30 minutes. After the doping process was completedthe temperature of the furnace was raised to a sintering temperature ofabout 1300° C. to complete the formation of a dense glass article. UsingX-ray fluorescence, the glass article was determined to have a chlorinecontent of be about 2.6 wt. %.

As exemplified, performing the doping process at higher pressuresenabled formation of a doped glass article with higher dopant content.Performing the doping process in a closed system such as describedherein enabled achieving the higher dopant content.

While the present disclosure has been described with respect to alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that other embodiments can bedevised which do not depart from the scope of the invention as disclosedherein. Accordingly, the scope of the present disclosure should belimited only by the attached claims.

1. A method of forming an optical element, the method comprising:producing silica-based soot particles using chemical vapor deposition,the silica-based soot particles having an average particle size ofbetween about 0.05 μm and about 0.25 μm; forming a soot compact from thesilica-based soot particles; and doping the soot compact with a halogenin a closed system by contacting the silica-based soot compact with ahalogen-containing gas in the closed system at a temperature of lessthan about 1200° C.
 2. The method of claim 1, wherein forming a sootcompact comprises depositing the silica-based soot particles onto a baitrod.
 3. The method of claim 1, wherein forming a soot compact comprisespressing the silica-based soot particles at a pressure of between about100 psi and about 1000 psi.
 4. The method of claim 3, wherein pressingthe silica-based soot particles comprises axially pressing thesilica-based soot particles to form a disc shaped soot compact.
 5. Themethod of claim 1, further comprising maintaining a predeterminedhalogen-containing gas composition in the closed system.
 6. The methodof claim 5, wherein maintaining a predetermined halogen-containing gascomposition in the closed system comprises bleeding halogen-containinggas into the closed system.
 7. The method of claim 1, wherein doping thesoot compact comprises maintaining a halogen composition in the closedsystem of greater than about 90% of the total gas composition of theclosed system.
 8. The method of claim 1, further comprising maintaininga predetermined halogen-containing gas partial pressure in the closedsystem.
 9. The method of claim 8, wherein the predeterminedhalogen-containing gas partial pressure in the closed system is betweenabout 0.10 atm and about 0.90 atm.
 10. The method of claim 1, whereinthe halogen-containing gas is a fluorine-containing gas.
 11. The methodof claim 1, wherein the halogen-containing gas is a chlorine-containinggas.
 12. The method of claim 1, further comprising consolidating thesoot compact in the closed system to form a glass article.
 13. Themethod of claim 12, wherein consolidating the soot compact is sufficientto form a glass article comprising a variation of halogen concentrationof less than about 0.20 wt. %.
 14. The method of claim 1, wherein thesilica-based soot particles comprise silica and titania.
 15. The methodof claim 1, wherein the silica-based soot particles have a surface areaof greater than about 10 m²/gram.
 16. A method of forming an opticalelement, the method comprising: producing silica-based soot particlesusing chemical vapor deposition, the silica-based soot particles havingan average particle size of between about 0.05 μm and about 0.25 μm;forming a soot compact from the silica-based soot particles; doping thesoot compact with a halogen in a closed system by contacting the sootcompact with a halogen-containing gas in the closed system at atemperature of less than about 1200° C.; and consolidating the sootcompact in the closed system to form a glass article by simultaneouslyincreasing the temperature in the closed system and decreasing theconcentration of the halogen-containing gas in the closed system. 17.The method of claim 16, wherein forming a soot compact comprisesdepositing the silica-based soot particles onto a bait rod.
 18. Themethod of claim 16, wherein forming a soot compact comprises pressingthe silica-based soot particles at a pressure of between about 100 psiand about 1000 psi.
 19. The method of claim 18, wherein pressing thesilica-based soot particles comprises axially pressing the silica-basedsoot particles to form a disc shaped soot compact.
 20. The method ofclaim 16, wherein decreasing the concentration of the halogen-containinggas in the closed system comprises decreasing the concentration of thehalogen-containing gas according to the following equation:${y_{II} = {y_{I,{dop}}{{Exp}\left\lbrack {{- 21741}\left( {\left( \frac{1}{T_{I,{dop}}} \right) - \left( \frac{1}{T_{II}} \right)} \right)} \right\rbrack}}},$wherein: T_(I,dop) is the temperature in the closed system when dopingthe soot compact with halogen; T_(II) is the temperature in the closedsystem when consolidating the soot compact; y_(I,dop) is theconcentration in mole fraction of the halogen-containing gas in theclosed system when doping the soot compact with halogen; and y_(II) isthe maximum concentration in mole fraction of the halogen-containing gasin the closed system when consolidating the soot compact at T_(II). 21.The method of claim 16, further comprising annealing the glass articleby cooling the glass article in the closed system at a temperature ofbetween about 900° C. and about 1100° C. and maintaining the temperaturebetween about 900° C. and about 1100° C. for less than about 10 hours.22. The method of claim 21, wherein annealing the glass article issufficient to form a glass article having a fictive temperature of lessthan about 1100° C.
 23. The method of claim 16, further comprisingdecreasing the temperature of the closed system to between about 700° C.and about 850° C. at a rate of less than about 10° C. per hour.
 24. Themethod of claim 16, wherein doping the soot compact further comprisesmaintaining a predetermined halogen-containing gas composition in theclosed system.
 25. The method of claim 16, wherein doping the sootcompact comprises maintaining a halogen composition in the closed systemof greater than about 90% of the total gas composition of the closedsystem.
 26. The method of claim 16, wherein doping the soot compactfurther comprises maintaining a predetermined halogen-containing gaspartial pressure in the closed system.
 27. The method of claim 26,wherein the predetermined halogen-containing gas partial pressure in theclosed system is between about 0.10 atm and about 0.90 atm.
 28. Themethod of claim 16, wherein the halogen-containing gas is afluorine-containing gas.
 29. The method of claim 16, wherein thehalogen-containing gas is a chlorine-containing gas.
 30. The method ofclaim 16, wherein consolidating the soot compact is sufficient to form aglass article comprising a variation of halogen concentration of lessthan about 0.20 wt. %.
 31. The method of claim 16, wherein thesilica-based soot particles comprise silica and titania.
 32. The methodof claim 16, wherein the silica-based soot particles have a surface areaof greater than about 10 m²/gram.